Conventionally, as a technology for storing electric energy, electric double layer capacitors (for example, see Patent Document 1) and secondary batteries have been known. Electric double layer capacitors are much superior to secondary batteries in terms of lifetime, safety and power density. However, the electric double layer capacitors have a problem in that the energy density (volumetric energy density) is lower than that of the secondary batteries.
The energy (E) stored in the electric double layer capacitor is expressed as E=½×C×V2 using the capacitance (C) and applied voltage (V) of the capacitor, and thus the energy is proportional to the capacitance and the square of the applied voltage. Therefore, in order to improve the energy density of the electric double layer capacitor, techniques for improving the capacitance and applied voltage of the electric double layer capacitor have been proposed.
As a technique for improving the capacitance of the electric double layer capacitor, a technique for increasing the specific surface area of the activated carbon constituting the electrode of the electric double layer capacitor has been known. At present, the known activated carbon has a specific surface area of 1,000 m2/g to 2,500 m2/g. In an electric double layer capacitor using such activated carbon as an electrode, an organic electrolyte solution obtained by dissolving a quaternary ammonium salt in an organic solvent, an aqueous electrolyte solution such as sulfuric acid or the like is used as an electrolytic solution.
Since the organic electrolyte solution has a wide usable voltage range, the applied voltage can be increased and the energy density can be improved.
A lithium ion capacitor utilizing the principle of an electric double layer capacitor has been known as a technique for improving the applied voltage of the electric double layer capacitor. A capacitor that uses graphite or carbon capable of intercalating and deintercalating lithium ions as a negative electrode and uses activated carbon equivalent to an electrode material of an electric double layer capacitor capable of adsorbing and desorbing electrolyte ions as a positive electrode is called a lithium ion capacitor. Further, a capacitor that uses activated carbon equivalent to an electrode material of an electric double layer capacitor as one of the positive electrode and negative electrode and uses a metal oxide or a conductive polymer as the other electrode, that is, an electrode where a Faradaic reaction occurs, is called a hybrid capacitor. In the lithium ion capacitor, among the electrodes constituting the electric double layer capacitor, the negative electrode is constituted of graphite, hard carbon or the like serving as a negative electrode material in a lithium ion secondary battery, and it is an electrode in which lithium ions are inserted within the graphite or hard carbon. Lithium ion capacitors have a characteristic in that the applied voltage is larger than that of general electric double layer capacitors, that is, those in which both electrodes are constituted of activated carbon. However, when graphite is used for the electrode, there is a problem in that propylene carbonate cannot be used as an electrolytic solution. When graphite is used for the electrode, propylene carbonate is electrolyzed, and the electrolyzed product of propylene carbonate adheres to the surface of the graphite, thereby reducing the reversibility of lithium ions. Propylene carbonate is a solvent that can operate even at low temperatures. When propylene carbonate is applied to an electric double layer capacitor, the electric double layer capacitor can operate even at −40° C. Accordingly, in lithium ion capacitors, hard carbon in which propylene carbonate is difficult to be electrolyzed is used for electrodes. However, hard carbon has lower capacity per volume of electrode as compared with graphite, and the voltage is also lower than that of graphite (becomes a noble potential). Therefore, there is a problem in that the energy density of the lithium ion capacitor is lowered.